The present invention relates to industrial process control and monitoring systems of the type used to measure and control industrial processes. More specifically, the present invention relates to measurement of flow in industrial processes based upon a differential pressure.
Process variable transmitters are used in industrial processes to measure process variables of various process fluids. Example process variables include pressure, temperature, level or height of process fluid in a tank, flow rate, pH, etc. These measurements can be used to monitor operation of the process, and can be further used as the basis of controlling the process. The transmitters are typically located at a remote location, and transmit the measured processed variable to a centralized location.
One technique used to measure flow in industrial processes is based upon a differential pressure. A restrictive element is placed within the flow, for example a smaller orifice in a process pipe. This creates a differential pressure across the restriction which is related to the flow rate. A differential pressure sensor can be used to measure the differential pressure across the restrictive element. This differential pressure can be correlated with flow rate by the transmitter, or the raw data can be transmitted to a centralized location for processing. In another configuration, rather than using a differential pressure sensor, two individual absolute or gage pressure sensors are used. The two measured pressures can then be subtracted, for example using digital circuitry, to determine the differential pressure.
In a flow measure system which uses two pressure sensors to determine differential pressure, errors may be introduced if one of the pressure sensors fails, or if its accuracy drifts significantly. Such a failure may cause the process to be incorrectly controlled, or may result in incorrect data being collected.